1. Field of the Invention
The present invention relates to the manufacture of paper cover sheet material for use in producing gypsum wallboard, and to the gypsum wallboard produced from the paper cover sheet material, and more particularly refers to such paper cover sheets in which there is incorporated mineral fiber.
2. Description of the Prior Art
As is known, paper covered gypsum board comprising a cast gypsum core and paper cover sheets is widely used in building construction. The product may be in the form of wallboard, lath and the like. In manufacturing such gypsum board the face paper is usually drawn over a forming table, a water-stucco slurry spread over the paper sheet and the back paper cover sheet applied thereover before the slurry has set. The board is then cut to desired size and dried in a kiln. In use the board is cut to size by scoring and snapping or by sawing, and is applied to a wall by means of clips, nails, screws or adhesives.
The strength and other properties of the finished gypsum board depend to a considerable extent on the paper cover sheets employed, which paper cover sheets must be of such nature as to have properties which enable the board to be manufactured to close dimensional specifications, be of high strength, have suitable surface quality, be readily dried, and able to form a good bond with the gypsum core.
It has been previously recognized that the incorporation of a minor proportion of mineral fibers into the cellulose furnish can result in the realization of improvement in the properties of the paper cover sheets. The use of mineral fiber is disclosed in U.S. Pat. No. 3,562,097 and U.S. Pat. No. 4,020,237. As disclosed in these patents, the paper cover sheet material is made by separately dispersing a mass of cellulose fibers and mineral fibers in water, combining the two dispersions of the fibers in desired proportion, and finally forming the combined fibers into a web of paper on a papermaking cylinder machine.
To reiterate, the advantages of blending mineral wool fibers with paper fibers are:
(1) improved stock drainage, PA0 (2) lower sheet porosity value, and PA0 (3) faster paper and board drying. PA0 1. The material shortening of the mineral fiber length required to achieve a wall dispersed slurry, and PA0 2. The existence of shot in the mineral wool fiber that settled out in the paper mill tanks and vats and, even when retained in the paper, deposited on the wires. PA0 3. Sheet tensile strength loss in papers containing mineral fibers.
Unfortunately, these advantages and the use of mineral fiber-containing paper have not been taken advantage of in the past because of three major problems. First and foremost, the use of mineral fibers results in an excessive amount of unretained shot contaminating the paper mill system. This results from the fact that conventional mineral fiber material as it is commonly produced has a large amount of shot, that is, spherical particles of molten and then hardened slag. Consequently, when the mineral fibers are dispersed in water, a great deal of energy is used in the dispersing process, during which a large amount of the shot is released and falls to the bottom of the apparatus and results in the contamination of the apparatus used in the papermaking process.
Second, a reduction in physical strength values has occurred in nearly a direct ratio to the amount of mineral fibers added. Finally, there has been poor dispersibility of the mineral fibers in the paper slurries, resulting in excessive shattering and shortening of the mineral fibers on mixing.